1. Field of the Invention
The present invention relates to alkalophilic methanogen and more particularly to Methanosarcina alcaliphilum having an optimum growth pH ranging from about 8.1 to about 8 7.
In addition, the present invention also relates to a methane fermentation method in which methanogens are used and more specifically to a method for carrying out methane fermentation at a high speed by increasing the bacterial concentration of methanogens in a reactor for methane fermentation within a short period, through the best use of the properties of the methanogens.
2. Description of the Prior Art
Heretofore, there have been known the following 5 bacterial species as methanogens belonging to genus Methanosarcina: Methanosarcina barkeri (see Microbiological Reviews, 1979, Vol. 43, pp. 260-296 and FEMS Microbiology Letters, 1983, Vol. 16, pp. 217-223); Methanosarcina acetivorans (see Applied and Environmental Microbiology, 1984, Vol. 47, No.5, pp. 971-978); Methanosarcina vacuolata (see Microbiology, U.S.S.R., 1979, Vol. 48, pp. 279-285); Methanosarcina thermophila (see International Journal of Systematic Bacteriology, 1985, Vol. 35, No.4, pp. 522-523); and Methanosarcina mazei (see Current Microbiology, 1980, Vol. 3, pp. 321-326).
However, not all of these methanogens are alkalophilic and their optimum growth pH condition is less than 7.5. There have not so far been discovered any bacteria having a high optimum growth pH in the order of not less than 7.5 to 8.0 and belonging to the genus Methanosarcina.
On the other hand, the following two bacterial species belonging to genus Methanobacterium have been known as alkalophilic methanogens: Methanobacterium alcaliphilum (see International Journal of Systematic Bacteriology, 1986, Vol. 36, No.3, pp. 380-382) and Methanobacterium thermoalcaliphilum (see Archives of Microbiology, 1985, Vol. 142, pp. 211-217).
However, both of these alkalophilic methanogens are rod-shaped belonging to genus Methanobacterium and they can utilize only hydrogen and carbon dioxide as substrates.
As described above briefly, alkalophilic methanogens discovered heretofore can simply assimilate hydrogen and carbon dioxide as the substrate and they belong to genus Methanobacterium or they have morphological properties of rods.
Besides, methane fermentation methods have been utilized widely for treating waste water and solid waste from the viewpoint of effective use of biomass, biogas production or the like.
Nevertheless, the conventionally known methanogens used in methane fermentation methods are strictly anaerobic bacteria and show extremely slow rate of growth. For this reason, the use of a large-scale reactor for methane fermentation is required and likewise it takes a long period of time to obtain bacterial concentration of the methanogens in the reactor which is required for maintaining a stable steady state of methane fermentation.
There has been proposed a method which comprises the step of controlling conditions in the reactor such as pH and temperature for the purpose of improving the efficiency of the foregoing methane fermentation. However, such a control of only the environmental conditions enables such a method to be speeded up only slightly and permits an improvement of efficiency only to a limited level.
In particular, in the methane fermentation methods for treating solid waste and/or waste water, main substrates are acetic acid and H.sub.2 /CO.sub.2 (among the substrates capable of being assimlated by methanogens, acetic acid and H.sub.2 /CO.sub.2 are exclusively present in such waste). However, the assimilation rate of acetic acid by methanogens is very low and this serves as a rate-determining step. Thus, it takes a long period of time to start the methane fermentation reactor and the volume of the reactor must be enlarged. Morever, the reactor is started according to adaptation-cultivation method using substrates inherent to the bacteria used.